RFC: "attempted dynamic environment-capture" is a bad error message

[bblum]

I propose replacing it with can't capture dynamic environment in a fn item; use the || { ... } closure form instead

Though, I note from #322 that this error message has been around since the days of rustboot. So maybe somebody has sentimental attachment to it...

[pcwalton]

Yes, this is a terrible error message. I wanted to change it when I rewrote resolve but I didn't want to update the compile-fail tests that relied on the error message being what it was.